Robust estimation of periodic artifacts is extremely difficult. A lot of research has been done to construct filters which only remove the periodic artifacts.
Most of these filters are not accurate at regions with high intensity gradients (edges). In these areas, most filters generate high responses.
The present invention specifically relates to an application in which an image signal is obtained by reading a radiation image that has been temporarily stored in a photo-stimulable phosphor screen. A digital signal representation of the stored radiation image is obtained by scanning the plate with stimulating radiation and converting the image wise modulated light which is emitted by the plate upon stimulation into a digital signal representation. The image-wise light emitted upon stimulation is focussed by means of an array of microlenses onto an array of transducers converting light into an electric signal.
Because an imaging plate such as a photo-stimulable phosphor screen has a varying thickness, several positions of the imaging plate are out of focus with respect to the microlens array.
After calibration of the received signal, the areas where the imaging plate was out of focus contain some periodic variation with the same period of the microlens array.
The period of a microlens array is defined as the width of one microlens in the microlens array.
When analyzing the Fourier spectrum of the received signal, peaks are observed in the Fourier spectrum at the frequency F of the microlens array and the harmonics, Fn=nF, n=1,2,3, . . . for a microlens with a width of T=1/F pixels. Many filters can be constructed to suppress periodic variation of this nature. However, they all have to be tuned carefully to ensure the removal of only the periodic variation.
In a similar manner, a filter can be constructed which extracts the periodic variation from the signal. This periodic estimation will contain extra erroneous information.